Brown adipose tissue (BAT) has long been suspected of playing an important yet undefined role in the regulation of energy balance. It is distinguished by the presence of uncoupling protein (UCP), a mitochondrial transporter which uncouples respiration. Because of this highly specialized capability, dysfunction of BAT could play a critical role in the pathogenesis of obesity. Previous studies have suggested such a link but definitive proof has been elusive. Using a transgenic approach, we have perturbed BAT first and subsequently examined the effect on systemic energy balance. This has been accomplished using a suicide DNA transgene through which the highly specific UCP promoter drives expression of diphtheria toxin A-chain (DTA). So far, utilizing the UCP-DTA transgene, we have generated five lines of mice which have defective BAT and extreme obesity. The original line, which has been partially characterized, possesses a variety of metabolic complications frequently associated with human obesity such as hyperinsulinemia, glucose intolerance and hypertriglyceridemia. Unexpectedly, these animals also develop late-onset, progressive hyperphagia suggesting the existence of a completely unsuspected link between BAT function, obesity and appetite regulation. Finally, unlike other genetic models of obesity and insulin resistance, the UCP-DTA mouse closely mimics the development of obesity in humans and as such may produce new insights into the development of these disorders. Studies proposed under the first specific aim will focus on BAT, its degree of dysfunction in UCP-DTA mice and the direct effect of this upon processes thought to be contributed to by BAT such as 24 hr. systemic energy balance, diet-induced thermogenesis and thermogenic response to beta-3 adrenergic agonists. Also, the effects of more complete ablation will be assessed in UCP-DTA mice which are homozygous for the transgene and additional UCP-DTA lines, one of which may express the transgene at a higher level. Finally, an attempt will be made to rescue UCP-DTA mice by transplanting syngeneic, brown fat preadipocytes from normal mice. Under the second aim, studies will focus on the complications of BAT ablation: expansion of fat and lean body mass, hyperinsulinemia, insulin resistance and hyperphagia. A longitudinal study will be performed in order to identify possible cause and effect relationships. Additional studies will focus on the mechanisms which underlie the development of insulin resistance and hyperphagia in this model. Work proposed under the third specific aim will concentrate on the generation of an additional line of mice using the UCP-HSVTK transgene. In these mice ablation will be induced by the administration of nucleoside analogs. By varying the time when ablation begins, the effects of BAT destruction at different ages will be assessed. Also, discontinuation of ablation and subsequent restoration of BAT will alllow the potential reversibility of the phenotype to be assessed in the absence of the initial insult.